16 August 2015

Flight tests of a Eurofighter Typhoon with a suite of aerodynamic modifications show they have measurably improved the fighter’s subsonic agility, say engineers and pilots involved in the project. The alterations can be easily retrofitted to any Typhoon, it is claimed, and have been demonstrated to pilots from Typhoon partner nations. The Aerodynamic Modification Kit (AMK) is being considered as part of future improvement packages for the aircraft.The AMK comprises small leading-edge root extensions (Lerxes), new delta-shaped fuselage strakes that replace the existing rectangular “step” strakes beneath the canopy sills, and larger flaperons (13% inboard, 8% outboard) that provide more forward sweep to the wings’ trailing edge. The primary function of the Lerx and reshaped fuselage strakes is to make vortex flows more stable and predictable as the angle of attack (AoA) increases. The larger flaperons provide more roll authority and also counteract the larger nose-up pitching forces introduced by the Lerxes and strakes. “We already have the most unstable production aircraft in the world,” one engineer says. “We don’t need any more pitch-up.”No new surfaces are added and the existing structure and flaperon actuators can cope with the changes, Eurofighter says, so the main tasks for transition to production would be the design of series-type AMK components and definition of a retrofit process.The effects of the AMK changes are dramatic, Eurofighter says. Operational AoA limits are increased by 45% and total lift by as much as 25%. The corner speed—defined as the lowest speed at which the aircraft can generate maximum g force, and hence the speed at which the aircraft achieves its highest turn rate and tightest radius—has been “significantly reduced,” increasing maximum turn rate. The usable pitch rate is increased “by far,” according to project test pilot Raffaele Beltrame, and roll rate by up to 100%. The largest improvements are at lower speeds, but performance and handling are also improved in the subsonic regime, and supersonic maneuverability—where the Typhoon is second only to the Lockheed Martin F-22—is unaffected.………At low speeds, limits on AoA, pitch rate and other parameters are often associated with vortices that form around the forebody and canard at high AoA, and which can cause severe lateral-directional instability if they develop or burst asymmetrically. Very small features toward the front of the aircraft can cause strong vortices.The step strakes on Typhoon were designed to generate stable vortices, based on computational fluid dynamics (CFD) technology available then. “Today we have much better tools,” Osterhuber says, and CFD is better integrated with wind tunnel testing. (The AMK configuration underwent extensive testing by the German-Dutch DNW wind tunnel consortium, and in Calspan’s transonic tunnel.) He also notes that, in the past few years, Airbus has improved the sharing of expertise and tools between its commercial and military divisions, including its work with government research institutions.

This is some impressive engineering.

This is a pretty impressive expansion of the flight envelope at a marginal cost of €0.00.